Depends on the scenario. If you're in the early stages of the Grand Campaign, you will be very low on funds until you make multiple conquests. France especially is crucial to the PP production of Germany. And all the minor countries that Germany conquered historically add PPs also.

In other situations, you must watch your social unrest. There is also a major upkeep increase if you expand your military too much (this reflects not only costs but manpower).

Watch your reports turn by turn on such things as upkeep and social unrest, and any events that may impact your production or saved PPs.